As “power transmission system” it is meant a system configured to deliver electricity to end users. To this purpose, a power transmission system may comprise electrical cables and joints and electrical apparatuses such as substations, transformers, disconnectors, switches, etc. The power transmission system may be either three-phase or single-phase and can carry power in the form of both direct current (DC) and alternating current (AC).
During operation, a power transmission system is typically subject to heating. Such heating is mostly due to the so-called Joule effect, according to which a current passing through a conductor having a certain resistance produces heat. The produced heat is proportional to the square of the current multiplied by the conductor resistance.
In order to prevent an undesired overheating of a power transmission system, its temperature should be kept under control by removing at least part of the heat produced by the passage of the current.
In particular, it is important to maintain the cable temperature below its top operating temperature. Over said limit, the cable performance is impaired due to resistive losses and the overall cable structure can be damaged by excessive heat.
The operation of removing heat is of particular importance when the power transmission system is at least partially laid down in a closed or partially closed environment, where air circulation is difficult or substantially absent. This is the case when the power transmission system is at least partially installed underground, for instance in a tunnel or in a manhole.
The temperature control of a power transmission system is typically performed by a cooling system comprising cooling devices (for instance, fans) placed at predetermined positions of the power transmission system. In particular, the cooling devices are typically placed in the vicinity of components of the power transmission system that are particularly prone to heating (e.g. due to a particularly high resistance), such as joints between electrical cables.
Granadino et al., Jicable '03, A.1.2, Jun. 22-26, 2003 describe a project for undergrounding a 400 kV power transmission line in a ventilated tunnel equipped with a forced cooling system. The forced cooling system comprises fan stations injecting fresh air inside the tunnel. The tunnel temperature is measured continuously by a Distributed Temperature Sensing (DTS) system and operation of the fans (with inverters to regulate the fan speed) is controlled by an automatic Real Time Thermal Rating (RTTR) system. This ensures that the boundary conditions on tunnel and cable temperatures are not exceeded.
GB 1,193,126 discloses an electric cable or bus-bar installation for transmitting electric power, of the kind in which heat generated in a load carrying conductor or conductors is dissipated by the circulation of a cooling fluid. The installation incorporates means for circulating or assisting the circulation of the cooling fluid, driven by an electric motor deriving all or part of its power supply from the secondary winding of at least one current transformer, the primary winding of which is a load carrying conductor forming part of the installation. The rate of circulation of the cooling fluid hence automatically varies in proportion to the load carried by the installation and may be caused to cease altogether when, owing to a reduction or cessation of the load, no artificial cooling is required.
The Applicant observes that the known cooling systems exhibit some drawbacks.
A cooling system like that described by Granadino et al. requires a complex installation, because dedicated circuits (namely, low voltage lines and inverters) are to be laid down in the tunnel for powering the fans stations. This also results in an increased cost of the system. Furthermore, such cooling system is not safe. Indeed, in case of a failure affecting the low voltage circuit supplying the fan stations and not the power transmission line, the cooling system stops operating, while the power transmission line is still operating and hence is still producing heat. In such situation, the temperature of the power transmission line cannot be controlled. The power transmission line may then overheat, which causes wear of the power transmission line and may even lead to hazardous events such as fires.
A cooling system like that disclosed by GB 1,193,126 is not economically efficient when the electric cable is carrying a reduced load. Although cooling a cable in general leads to a decrease of its resistance, and accordingly to a decrease of the power loss of the cable (which in turn results in a decrease of the cost for transporting power through the cable), in case of very reduced loads the cost decrease associated to the cable resistance reduction may be lower than the cost associated to the operation of the cooling system. As a consequence, circulating a cooling fluid in a rate proportional to the load carried by the installation results in an increase of the overall operational cost of the installation when the cable is carrying a reduced load.
On the other side, the Applicant observed that also ceasing the circulation of the cooling fluid owing to a reduction of the cable load surprisingly brings to an increase of the overall operational cost of the installation.
In view of the above, the Applicant has tackled the problem of providing a cooling system and a method for cooling a power transmission system (in particular, but not exclusively, a high voltage power transmission system), which overcomes the aforesaid drawbacks.